THE ONTOGENESIS OF VERTEBRATES 53 



are in condition to enter the egg. It is also likely that here, 

 as in many other cases, several spermatozoa may actually 

 enter the egg substance, but that all except one are simply 

 added to the yolk and serve as food.* 



The spermatozoon, after the entrance into the egg is once 

 effected, drops its locomotor apparatus and becomes merely 

 a nucleus, which fuses with the one belonging to the egg, a 

 procedure similar to conjugation in the Protozoa. The egg 

 cell thus becomes furnished with a fusion-nucleus, and may 

 be considered from now on the first cell of a new organism. 

 From it arise all the cells of the developing animal through 

 the process of fission, and, since a division of the nucleus al- 

 ways precedes the division of the cell, it follows that this 

 fusion-nucleus is in the same way the primary one from which 

 all later nuclei are to be derived. Since now, as has been shown 

 bv direct observation, this fusion-nucleus becomes divided 

 in such a way as to effect an exactly equal division of both 

 maternal and paternal components, and since the process has 

 been found to continue as far as the investigators have been 

 c.ble to follow it, it is extremely probable that the nucleus of 

 each and every cell of the adult organism contains an element 

 derived from each of its parents. 



Herein lies a material basis for the phenomena of heredity, 

 and it thus becomes evident that all hereditary traits and char- 

 acters are perpetuated through the direct transmission and 

 growth of a bit of material furnished by each parent and 

 handed doii'n to each cell of the organism. Although this is 

 still the mystery of mysteries to the biologist, the careful study 

 of the past twenty or thirty years, directed upon this very 

 point, has revealed much, but in so doing has added more 



* Until 1875 it was generally supposed that more than one sperma- 

 tozoon took part in the fertilization of an egg. The true facts in the 

 case were first determined by observation, and later proven by direct 

 experiment. Polyspermy, or the introduction into the egg of more than 

 one spermatozoon, has been experimentally brought about in the eggs of 

 various marine animals by such methods as the application of heat and 

 cold or the use of poisons, and in all cases the resulting development has 

 been abnormal. 



